The invention relates generally to gate fully controlled thyristors controlled by a drive circuit, including a turn-on circuit and a turn-off circuit, and more specifically to a method and system for protecting such gate fully controlled thyristors from unacceptable overvoltage.
The journal "Elektronik", No. 20, of Oct. 5, 1984, pages 111 to 115, and the journal "Elektronik", No. 21, of Oct. 19, 1984, pages 91 to 96, discloses an optimized drive circuit, consisting of a turn-on circuit and a turn-off circuit, for gate fully controlled thyristors (Gat e-Turn-Off thyristors or GTO thyristors), in particular gate fully controlled power thyristors. The first-named article discusses the turn-on circuit, the second article discusses the turn-off circuit.
The journal "Elektroniker", No. 5, 1984, pages 49 to 56, in particular page 52, discloses a control generator for a high power GTO thyristor. This control generator contains a turn-on circuit, a turn-off circuit, and a blocking circuit which supplies the gate of the high-power GTO thyristor with a negative gate voltage during the blocking phase. The control generator is constructed with power metal oxide semiconductor field effect transistors (power MOSFET).
The journal "BBC-Nachrichten", 1977, No. 3/4, pages 152 to 158, discloses protective measures for a thyristor. Voltage-limiting elements, as e.g. varistors (silicon-carbide varistor, oxide varistor), two anti-series connected surge-proof silicon diodes (BBC-DASAS elements) or breakover diodes (BOD) are used as protective elements BOD elements are thyristors dimensioned to defined breakover voltages without an external gate terminal They ignite by overshoot of the null breakover voltage and fire the thyristor to be protected to whose anode-gate path a BOD element is connected in parallel.
This protective measure, where a BOD element with a series resistor links the anode of the thyristor to be protected with its gate, cannot be used as protective measure for GTO thyristors as the GTO thyristor is kept in the blocked state by a negative gate voltage. With the use of a BOD element, as in the overvoltage protection of a thyristor, the current through the BOD element would, in response to overvoltage, flow off into the negative voltage source and would not become effective as ignition current. That is, the overvoltage protection constructed with a voltage-limiting element, connected in parallel to the anode-gate path of the GTO thyristor, as in the overvoltage protection for thyristors, would not be able to protect a GTO thyristor against overvoltages.
Therefore, there is a need to develop a method and system for protecting a GTO thyristor against unacceptable overvoltage.